The present invention pertains generally to the field of optical lithographic systems. More specifically, the present invention relates to resolving a mask""s image on a semiconductor wafer.
In the semiconductor industry, the ongoing quest of integrating more transistors onto a microchip requires transistors to have smaller critical dimensions. Consequently, in order to achieve a smaller critical dimension for these transistors, mask features to be exposed onto a wafer would need to be made smaller as well. However, as light passes through these finer mask features, light containing critical information for resolving the mask image on the wafer diffracts away from the mask in a larger diffraction angle. As this diffraction angle becomes larger due to light passing through finer mask features, the diffracted light travels beyond the reduction lens. Hence, the diffracted light will not be captured by the reduction lens and will not arrive at the wafer. Consequently, the mask feature information contained in the diffracted light is unavailable for resolving the mask image on the wafer. Thus, as the diffraction angle increases due to finer mask features, a need exists for retaining the information lost through the diffracted light in order to utilize the information contained in the diffracted light for resolving the mask feature image on the wafer.
In one prior art attempt to resolve finer mask feature image on the wafer by retaining the information contained in the diffracted light, an enlarged lens is manufactured to capture the diffracted light traveling beyond the reduction lens. However, the size of the reduction lens cannot be increased indefinitely. In fact, as is well known in the art, as lithography technology progresses into the sub-micron level, the size of the reduction lens is approaching its maximum limit.
In another prior art attempt to solve the problem of losing the information contained in the diffracted light, the mask itself is altered to produce a phase shifted mask, wherein contrast of the aerial image generated from the phase shifted mask is increased. In doing so, the phase shifted mask decreases light diffraction from the mask, thereby decreasing the amount of the diffracted light traveling beyond the reduction lens. Consequently, information which would have been lost in the diffracted light becomes available for resolving finer mask features on the wafer. However, a drawback of the phase shifted mask is the resulting extremely high manufacturing cost. Therefore, accommodating smaller mask features and designs by manufacturing shifted masks is not cost effective.
In yet another prior art attempt to retain the information lost through light diffraction, off-axis illumination technique is used to reduce the light diffracting beyond the reduction lens. This technique requires tilting the light source at various angles to illuminate the mask. However, this technique requires complicated and precise positioning of the light source. This precise positioning greatly reduces the margin for errors in the lithographic process.
Thus, a need exists for a system and method for enhancing an optical lithography process where the system and method does not require the use of an enlarged lens in capturing the diffracted light traveling beyond the reduction lens in order to retain information necessary for resolving the mask image on the wafer. A further need exists for a system and method for enhancing an optical lithography process where the system and method does not require the use of a phase shifted mask in reducing light diffraction in order to retain information necessary for resolving the mask image on the wafer. Still a further need exists for a system and method for enhancing an optical lithography process where the system and method does not require the use of off-axis illumination technique in reducing the diffracted light traveling beyond the reduction lens in order to retain information necessary for resolving the mask image on the wafer.
The present invention is a system and method for enhancing an optical lithography process by capturing light diffracted from a mask having features to be exposed onto a wafer. The present invention advantageously avoids the use of a physically unfeasible enlarged lens to capture diffracted light from the mask. Also, the present invention advantageously avoids the use of a very expensive phase shifted mask to reduce the information loss through the diffracted light. Furthermore, the present invention advantageously avoids the use of complicated off axis illumination technique in reducing the diffracted light traveling beyond the reduction lens.
Specifically, in one embodiment, a system of the present invention has in place a mask, a wafer and a reduction lens such that the reduction lens is placed between the mask and the wafer in order to direct and expose the mask""s features onto the wafer. Furthermore, a reflective member is disposed proximate to the reduction lens. In order to achieve finer resolution of the mask image on the wafer, this reflective member captures diffracted light diffracting beyond the reduction lens and redirects the diffracted light to pass through the reduction lens such that the diffracted light is redirected onto the wafer. In so doing, the reflective member resolves the mask image on the wafer in more detail than is possible by an optical lithography process using no such reflective member.
In another embodiment of the present invention, an optical lithography system has in place a mask, a wafer and a reduction lens disposed between the mask and the wafer. In addition, the system has a reflective member comprised of two reflective surfaces, wherein the first reflective surface is disposed lower than the mask and higher than the reduction lens, and wherein the second reflective surface is disposed lower the reduction lens and higher than the wafer. The first reflective surface captures diffracted light diffracting beyond the reduction lens and redirects the diffracted light to pass through the reduction lens to reach the second reflective surface. The second reflective surface captures the diffracted light emerging from the reduction lens and redirects the emerging diffracted light onto the wafer to resolve the mask image. Furthermore, the orientation of each reflective surface can be adjusted according to specific image geometry of the mask. Consequently, in comparison to an optical lithography system having no such reflective member, this embodiment of the present invention resolves the image geometry of the mask in finer details.
In yet another embodiment of the present invention, in addition to a mask, a wafer and a reduction lens placed between the mask and the wafer, a reflective member is disposed proximate to the reduction lens. Furthermore, this reflective member comprises two reflective surfaces, wherein each reflective surface is formed by a series of mirrors. The first reflective surface formed by a first series of mirrors is placed lower than the mask and higher than the reduction lens. This first series of mirrors is calibrated to capture diffracted light from the mask and redirect the diffracted light to pass through the reduction lens. The second reflective surface formed by a second series of mirrors is placed lower than the reduction lens and higher than the wafer. This second series of mirrors is calibrated to capture the diffracted light emerging from the reduction lens and redirect the diffracted light toward the wafer. By capturing the information contained in the diffracted light not going through the reduction lens and using this information to resolve the mask image on the wafer, this embodiment of the present invention achieves a resolution of the mask image on the wafer not possible with an optical lithography apparatus having no such reflective member.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.